Gypsum

Gypsum
Desert rose, 10 cm long
General
Category	Mineral
Chemical formula	Calcium Sulfate CaSO4·2H2O
Identification
Color	White to grey, pinkish-red
Crystal habit	Massive, flat. Elongated and generally prismatic crystals
Crystal system	Monoclinic 2/m
Twinning	common {110}
Cleavage	good (66° and 114°)
Fracture	Conchoidal, sometimes fibrous
Mohs Scale hardness	1.5-2
Luster	Vitreous to silky, pearly, or waxy
Refractive index	α=1.520, β=1.523, γ=1.530
Optical Properties	2V = 58° +
Pleochroism	None
Streak	White
Specific gravity	2.31 - 2.33
Fusibility	3
Solubility	hot, dilute HCl
Diaphaneity	transparent to translucent
Major varieties
Satin Spar	Pearly, fibrous masses
Selenite	Transparent and bladed crystals
Alabaster	Fine-grained, slightly colored

Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO₄·2H₂O.^[1]

Crystal varieties

Gypsum from New South Wales, Australia

Gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite. It may also occur silky and fibrous, in which case it is commonly called satin spar. Finally it may also be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or lightly-tinted variety of gypsum is called alabaster, which is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form typically opaque with embedded sand grains called desert rose. The most visually striking variety, however, is the giant crystals from Naica Mine. Up to the size of 11m long, these megacrystals are among the largest crystals found in nature. A recent publication shows that these crystals are grown under constant temperature such that large crystals can grow slowly but steadily without excessive nucleation.^[2]

Occurrence

Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. Deposits are known to occur in strata from as early as the Permian age.^[3] Gypsum is deposited in lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near surface exposures. It is often associated with the minerals halite and sulfur.

Fibrous Gypsum from Brazil

The word gypsum is derived from the Greek word γύψος, "chalk" or "plaster"^[4]. Because the gypsum from the quarries of the Montmartre district of Paris has long furnished burnt gypsum used for various purposes, this material has been called plaster of Paris. It is also used in foot creams, shampoos and many other hair products. It is water-soluble.

Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions of the White Sands National Monument in the US state of New Mexico have created a 710 km² (275 square mile) expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.^[5] Commercial exploitation of the area, strongly opposed by area residents, was permanently prevented in 1933 when president Herbert Hoover declared the gypsum dunes a protected national monument.

Commercial quantities of gypsum are found in Jamaica, Iran, Thailand, Spain (the main producer in Europe), Germany, Italy, England, Ireland, in British Columbia, Manitoba, Ontario, Nova Scotia and Newfoundland in Canada,^[6] and in New York, Michigan, Indiana^[6],Texas(in the Palo Duro Canyon),Iowa, Kansas, Oklahoma, Arizona, New Mexico, Colorado, Utah and Nevada in the United States. There is also a large mine located at Plaster City, California in Imperial County, and in East Kutai, Kalimantan.

Vast crystals of gypsum up to 10 metres in length have been found in the "Cueva de los Cristales" in the Naica Mine in the Mexican state of Chihuahua.^[7]

Uses of Gypsum

There are a large number of uses for gypsum throughout prehistory and history. Some of these uses are:

Cones of gypsum which formed on the sea floor during the Messinian salinity crisis

• Drywall
• Plaster ingredient.
• Fertilizer and soil conditioner. In the late eighteenth and early nineteenth century, Nova Scotia gypsum, often referred to as plaister, was a highly sought fertilizer for wheat fields in the United States.
• A Binder in Fast-Dry tennis court clay.
• Plaster of Paris (surgical splints; casting moulds; modeling).
• A wood substitute in the ancient world; for example, when wood became scarce due to deforestation on Bronze Age Crete, gypsum was employed in building construction at locations where wood was previously used.^[8]
• A tofu (soy bean curd) coagulant, making it ultimately a major source of dietary calcium, especially in Asian cultures which traditionally use few dairy products.
• Adding hardness to water used for homebrewing.
• Blackboard chalk.
• A component of Portland cement used to prevent flash setting of concrete.
• Soil/water potential monitoring (soil moisture tension)
• A medicinal agent in traditional Chinese medicine called Shi Gao.

References

External links

• WebMineral data
• Mineral Data from Mindat
• Mineral Galleries- Gypsum
• U.S. and National Gypsum Co. mines, Shoals, Martin Co., Indiana, USA